Exposure to A/PR/8/34 and A/FM/1/47 induces protective immunity against A/California/04/09. Cohorts of BALB/c mice were sublethally infected intranasally with 0.1 × LD50 live mouse-adapted A/PR/8/34 and A/FM/1/47 influenza viruses. We collected sera from these mice at days 7, 14, and 28 and determined their A/California/04/09-specific IgG titers (A) and HAI titers (B). One month later, these mice were challenged with 3 × LD50 live mouse-adapted A/California/04/09 virus and monitored for survival up to 14 d postchallenge (C). Body weight changes, indicative of morbidity, were also monitored up to 14 d (D). The graphs represent the mean ± SEM.